1. Field of the Invention
The present invention relates to a plasma processing system, and more particularly, relates to an antenna supplying a large power and useful for generation of high density plasma without causing any loss and a plasma processing system efficiently generating high density plasma using the antenna and performing predetermined processing on the surface of a substrate.
2. Description of the Related Art
Among the systems for performing predetermined processing on the surface of a semiconductor wafer or liquid crystal substrate (hereinafter referred to as a xe2x80x9csubstratexe2x80x9d) using plasma, plasma enhanced chemical vapor deposition (PCVD) and plasma etching systems are widely known. In these plasma processing systems, it is necessary to generate high density plasma in order to increase the processing rate. In addition, from the viewpoint of preventing impurities, it is required to form high density plasma by a lower pressure.
To generate plasma for the surface processing, from the viewpoint of obtaining high density plasma with a high efficiency, a system using the gaseous discharge generated by high frequency power is used. The inventors of the present patent application have already proposed a plasma processing system of a type supplying a high frequency power of 2.45 GHz to a radial slotted antenna connected to a coaxial high frequency power feed system to generate plasma (Japanese Patent No. 8-2534219) and have confirmed that good plasma processing was possible (as document, see for example N. Sato et al., xe2x80x9cUniform Plasma Produced by a Plane Slotted Antenna With Magnets For Electron Cyclotron Resonancexe2x80x9d for the configuration of a plasma processing system using a slotted antenna shown in the above document. This plasma processing system has a vacuum chamber 102 provided with an evacuating mechanism 101 and generating a discharge inside for generation of plasma, an antenna device 104 arranged on the upper section of the vacuum chamber 102 and provided with a slotted antenna 103, a high frequency wet-feed system 105 for feeding high frequency power to the slotted antenna 103, a discharge gas introduction mechanism 105 for introducing a discharge gas into the vacuum chamber 102, and a substrate holder 107 arranged at a lower position inside the vacuum chamber 102. A substrate 108 is loaded on the substrate holder 107 as an object to be processed. The shape of the slots (or slits) formed in the slotted antenna 103 is explained in detail in the above-mentioned patent specification or document. The slotted antenna 103 is actually provided with a magnetic circuit formed by permanent magnets etc. for generating a magnetic field near the electromagnetic wave emitter 103a, but in FIG. 9, its illustration is omitted. Further, as a result of the addition of the magnetic circuit, the slotted antenna 103 originally to be produced as the disk-shaped conductor plate is actually produced as a conductor having a predetermined thickness being able to house a magnetic circuit. In FIG. 9, however, for convenience of explanation, it is shown as a plate material. The high frequency power feed system 104 supplying the high frequency power is comprised of a high frequency power source 111, a stub tuner 112, a coaxial waveguide converter 113, a coaxial line 114, and a coaxial vacuum window 115.
The substrate 108 loaded on the substrate holder 107 is arranged to face the electromagnetic wave emitter 103a in the slotted antenna 103.
In the plasma processing system shown in FIG. 9, the vacuum chamber 102 is evacuated by the evacuating mechanism 101, discharge gas is introduced into the vacuum chamber 102, and a predetermined high frequency power is supplied to the slotted antenna 103 by the high frequency power feed system 105. The introduced discharge gas starts to discharge by the high frequency wave emitted from the electromagnetic wave emitter 103a of the slotted antenna 103 and generates plasma in the space in front of the substrate 108 in the vacuum chamber 102. The surface of the substrate 108 is processed by the physical or chemical action of the generated plasma. For example, if gas having an etching action is introduced as the discharge gas, the surface of the substrate 108 is etched.
Note that in the above-mentioned related art, an industrial frequency of 2.45 GHz is used as the frequency of the high frequency power. Further, the flux density of the magnetic field generated near the antenna by the magnetic circuit, corresponding to the high frequency, is set to be larger than about 875 Gauss so that the frequency of the electron cyclotron becomes equal to 2.45 GHz.
In the field of art of general antennas for transmitting an electromagnetic wave of the microwave to the millimeter wave band, conventionally, the folded waveguide proposed In Japanese Unexamined Patent Publication (Kokai) No. 9-199901 is known. This folded waveguide was proposed to solve the problem of the conventional folded waveguide shown in FIG. 14 of Japanese Unexamined Patent Publication (Kokai) No. 9-199901, that is, the need for formation of reflection surfaces of 45 degrees cuts at the top and bottom of the folded ends and the attachment of adjustment screws for canceling out reflection waves at the reflection surfaces and the resultant complexity of the configuration, the requirement for high dimensional precision, the high cost and inability of mass production, the narrow band of the frequency characteristics, and the troublesome adjustment work. Therefore, the folded waveguide proposed in Japanese Unexamined Patent Publication (Kokai) No. 9-199901 is characterized, as defined for example in claim 1 and claim 2, by setting an xe2x80x9chxe2x80x9d satisfying predetermined conditions in the dimensions axc3x97h (shown in FIG. 1) of the opening window of the 180 degrees folded portion.
In general the substrates processed by plasma processing systems have become larger in size in recent years. In the process of production of an LSI by processing of a silicon substrate, it is necessary to fabricate a large number of devices from a single substrate, so the size of substrates have become larger. Therefore, the above-mentioned plasma processing systems have been required to be increased in the power of the high frequency wave supplied in order to make the area of the plasma generation region (area of plane parallel to the substrate) larger and to make the plasma density higher for increasing the processing rate.
The antenna device 104 comprised of the above slotted antenna 103 is predicated on the processing of a substrate of a diameter of about 200 mm using plasma of a density of 1011 cmxe2x88x923 or so generated by the supply of a high frequency power of about 1 kW. Therefore, it is not possible to supply a large power high frequency wave outside of this assumption and therefore not possible to generate high density plasma suited to the processing of a large area substrate. The reason why a large power high frequency wave cannot be supplied is that a standing wave is generated due to the mismatch of the impedance at the high frequency wave propagation path formed in the slotted antenna 103 and therefore a locally strong electrical field is generated and causes insulation breakdown. Further, the electrical field induced in the slotted antenna 103 due to the standing wave becomes large and the surface of the slotted antenna 103 is heated by the Joule effect resulting in a loss of power which in turn obstructs the realization of a higher density plasma. In this slotted antenna, it is generally impossible to avoid mismatch of impedance arising due to the discontinuity in the shape of the high frequency wave propagation path.
Further, according to the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-199901 explained above, it is made possible to match the impedance without adjustment in the folded waveguide of a low loss transmission line of an electromagnetic wave of the microwave to the millimeter wave band and thereby eliminate the reflection wave and thus eliminate the standing wave. This technology, however, is limited to a folded waveguide comprised of the wide area surface of a rectangular waveguide folded substantially 180 degrees. When the width of the wide area surface is made xe2x80x9caxe2x80x9d and the width of the narrow wall surface is xe2x80x9cbxe2x80x9d, these dimensions xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d may be used to give conditions for eliminating the standing wave. Therefore, this technology mainly relates to the structure of the folded portion of a rectangular waveguide and does not relate to an antenna structure. Further, the above publication alludes to a folded radial waveguide (circular waveguide) in its eighth embodiment (FIG. 12 and paragraph 0049 etc.) and claims 12 and 13 as a modification of a folded waveguide. In this case, the folded radial waveguide uses 2xcfx80r (xe2x80x9crxe2x80x9d being the distance from the center of the radial waveguide 61 to the center position of the opening of the folded waveguide 64) as the value corresponding to the width xe2x80x9caxe2x80x9d of the wide area surface. It is possible to realize a plane array antenna using the folded radial waveguide, but this is only a modification of the folded waveguide satisfying the predetermined conditions in the end.
In particular, in an antenna used in the above plasma processing system, since a magnetic circuit is provided for forming a magnetic field of a predetermined distribution in the plasma generation space, in actuality a space for accommodating the magnetic circuit is provided and a disk-shaped conductor having a predetermined thickness is used. When using the antenna comprised of the disk-shaped conductor having the above thickness to supply a high frequency power into the vacuum chamber for the processing of the substrate, it is extremely difficult to have the most suitable impedance matching. For the impedance matching and efficient propagation of a high frequency wave without causing a standing wave, a new concept of antenna design suitable for the type and structure of the antenna is required.
An object of the present invention is to make improvements to the structure of a plasma generation antenna comprised mainly of a disk-shaped conductor having a predetermined thickness and provided with an electromagnetic emitter, while proposing an innovative antenna design technique, and thereby provide an antenna able to prevent the generation of a standing wave in a high frequency wave propagation path and generate high density plasma by the supply of a large power.
Another object of the present invention is to provide a plasma processing system being able to use the antenna to supply a large power high frequency wave, generating high density plasma by a large power, and processing the surface of a large area at a high rate.
The plasma processing system according to the present invention is configured as follows so as to achieve the above objects.
The plasma processing system of the present invention has, as a presupposition configuration, a vacuum chamber in which plasma is generated in a space at the front of a substrate arranged therein, an antenna for plasma generation provided in the vacuum chamber, and a high frequency power source for supplying high frequency power to the antenna. The antenna supplied with the high frequency power from the high frequency power source emits the high frequency power to cause generation of plasma in the space in the vacuum chamber. The plasma is used to perform predetermined processing of the surface of the substrate. Further, in the plasma processing system, the antenna has a disk-shaped conductor having a predetermined thickness and an electromagnetic emitter facing the substrate. It is connected to the high frequency power source by a coaxial line or cable. The disk-shaped conductor is connected to an inside conductor of the coaxial line at its center point. A waveguide of a coaxial type arranged symmetrically with respect to the center point and provided with a folded portion from the coaxial line to the electromagnetic emitter is provided around the disk-shaped conductor. The folded portion of the waveguide is structured as a short-circuit 3 dB directional coupler for impedance matching.
The above-mentioned plasma processing system has a radial waveguide including the disk-shaped conductor having the predetermined thickness due to housing a magnetic circuit and including the folded portion around it. The high frequency power supplied from the top side of the disk-shaped conductor is propagated to the electromagnetic wave emitter at the bottom side through the radial waveguide and is emitted from the electromagnetic wave emitter to the space inside the vacuum chamber. In the antenna having this structure, the waveguide is given the structure of a short-circuit 3 dB directional coupler. This is used for impedance matching to prevent generation of a standing wave.
Among antennas for supply of the high frequency power used in plasma processing systems, there has never before been an antenna having a disk-shaped conductor having a predetermined thickness which can perform impedance matching. According to the present invention, structure enabling impedance matching is realized by this new antenna design technique.
In the plasma processing system according to the present invention, preferably the structure of a short-circuit 3 dB directional coupler is obtained by forming a step difference at one or both of the top surface and bottom surface of the disk-shaped conductor. The disk-shaped conductor having a three-dimensional shape forms a waveguide with the external chamber. The antenna is provided at, for example, the top of the vacuum chamber used as the processing chamber. The high frequency propagation conditions of the waveguide having the folded portion are changed by the formation of the step difference. The structure of the short-circuit 3 dB directional coupler is realized by providing a step difference meeting predetermined conditions regarding the three-dimensional shape of the disk-shaped conductor. Impedance is matched by the waveguide.
Further, in the above configuration, the structure of a short-circuit 3 dB directional coupler is given by providing a plurality of dielectric materials in the region of the waveguide formed around the disk-shaped conductor divided into for example smaller regions and adjusting the heights or dielectric constants of the dielectric materials to satisfy predetermined conditions.
Further, In the above antenna, the variables (dimensions, dielectric constant, etc. of parts) of any elements in the plurality of elements comprising that structure of a short-circuit 3 dB directional coupler are determined to give S22=xcex93A* (where xe2x80x9c*xe2x80x9d is a conjugated complex number) in the representation of the scattering matrix with respect to the reflection coefficient xcex93A of the antenna. This condition is one example of the predetermined conditions. There are various elements determining the scattering matrix in the above plurality of elements. Further, similarly, in the antenna, the variables of any elements in the plurality of elements comprising the structure of the short-circuit 3 dB directional coupler are determined to give S22=0 in the representation of the scattering matrix. This condition is another example of the predetermined conditions and is a basic condition with high practicality.
The plasma processing system according to the present invention is preferably provided with a magnetic circuit for generating a magnetic field in the space inside the disk-shaped conductor. By providing the magnetic circuit, the disk-shaped conductor is given a predetermined thickness. Since the disk-shaped conductor has the predetermined thickness, a new unique technique for antenna design or impedance matching is provided.
In the above configuration, the flux density of the magnetic field generated by the magnetic circuit in the region in proximity to the disk-shaped conductor In the space of the vacuum chamber is set so that the electron cyclotron frequency corresponding to the flux density becomes higher than the frequency of the high frequency power.
Further, in the above configuration, the frequency of the high frequency power is 0.5 to 10 GHz.
In the plasma processing system according to the present invention, preferably a coaxial type impedance matching mechanism is provided at the coaxial line connected to the antenna.
Note that in the above explanation, the explanation was made focusing on a plasma processing system provided with the new high frequency feed antenna, but the antenna itself is also highly valuable technically.
The present invention exhibits the following effects. It provides the plasma processing system supplying a high frequency power into the vacuum chamber to cause discharge and generate plasma and thereby process the surface of a substrate, when the disk-shaped conductor supplying high frequency power has the predetermined thickness, the waveguide surrounding the disk-shaped conductor is given the structure of a short-circuit 3 dB directional coupler. Thereby the generation of a standing wave can be prevented, the high frequency power can be transmitted efficiently, and the efficiency of plasma generation can be improved. Therefore, a large power high frequency wave can be supplied, a high density plasma can be generated, and the surface of a substrate of a diameter more than 300 mm can be processed. Further, according to the present invention, the effect is more remarkable when using discharge resulting from a high frequency power with a frequency in the range of 0.5 to 10 GHz to generate plasma with a good uniformity over a large area. It is possible to improve the practicality of the plasma processing system when processing a large area substrate by high frequency discharge.